The staged operation of a premix burner of the double-cone burner type has been described in publication WO-A1-2001/096785 or in the equivalent U.S. Pat. No. 6,769,903. The operating method which is disclosed in these publications enables a flexible operation of the burner with low emissions and pulsations over a wide range of fuel and environmental parameters.
A burner configuration for carrying out the method, which is described in the publications, is reproduced in FIG. 1. FIG. 1 shows a burner arrangement 10 in the form of a so-called double-cone burner which has a swirl chamber 17 which is enclosed by a double cone 11 (for the principle of operation of the double-cone burner see for example EP-A1-0 321 809). A fuel lance 12, from which liquid fuel 16 discharges at the front end in the axial direction and which is surrounded by screening air 15, projects into the swirl chamber 17 in the axial direction to the burner axis 21. A premix fuel is injected into the burner in two stages 13 and 14. In the first stage 13, the premix fuel is injected radially outwards from the central fuel lance 12 via a multiplicity of injection openings (indicated by the arrows in FIG. 1). The injection openings are formed and arranged so that a predetermined distribution of the injected premix fuel results.
In order to achieve a good mixing-through of the fuel with air, the penetration depth of the fuel jets into the crossing air flow must be sufficiently high. The penetration depth of a jet in a crossflow depends upon the diameter of the jet and upon the ratio of the impulse flows of jet and crossflow. In order to achieve an adequate penetration depth with a multiplicity of injection openings with small diameter, rather high injection velocities are required. This leads to special demands upon the level of the gas pressure for the first stage.
High gas pressures in the first stage represent a considerable problem for the existing premix burner arrangement. Consequently, limits are set on the use of multistage premix burners in systems with low gas pressure. It is theoretically possible to alleviate the problem by the gas pressure being increased by means of an additional compressor. However, as a result of this the costs for the installation and the operation are significantly increased. It is therefore desirable to find a solution in which the demands upon the gas pressure for the first stage are reduced by the burner configuration without the operation of the burner being consequentially impaired.